The present invention relates to FM-IF receiver systems and, more particularly, to a circuit and method for linearizing the output signal produced at the output of a quadrature detector of such systems.
The use of quadrature detectors/demodulators in FM receivers to translate a frequency modulated input signal to an amplitude modulated output signal is well known in the art. For example, U.S. Pat. No. 3,548,326 discloses a FM-IF receiver utilizing a quadrature detector. Typically, the quadrature detector comprises a multiplier or gating circuit which is responsive to two input signals. The first input signal is utilized as a reference or switching signal and is generated from the FM-IF limiter stage of the receiver preceding the multiplier. The second input signal is generated from the first input signal being passed through a phase shift network comprising a quadrature coil and capacitor tuned tank circuit. This LC tuned circuit provides the second input signal which is in quadrature phase with respect to the switching or reference signal. The two input signals produce an output signal at the differential outputs of the multiplier that is proportional to the phase difference therebetween. FIG. 1, herein, illustrates such a quadrature detector. In many applications the differential outputs of the multiplier circuit are connected to a differential-to-single ended conversion circuit for providing a single ended output signal. The quadrature detector output signal can then be integrated to produce the audio output signal.
A dominant source of distortion in many FM receiver systems arises from the use of the type of quadrature demodulators aforedescribed. This distortion is related primarily to the third harmonic frequency of the FM carrier frequency and is known as third harmonic distortion (THD). One source of THD is due to the use of a single tuned quadrature coil that is utilized in the phase shifting network of the detector.
One prior art FM-IF system, in an attempt to eliminate or reduce THD, utilizes a double tuned tank circuit. However, not only are additional components required by this system, but it is difficult to align in production assembly. Therefore, this system is not only costly but is undesirable for mass production of FM receiver systems. Another prior art system utilizes a single tuned quadrature coil and feedback in an attempt to reduce THD. In this system the output of the multiplier is supplied to a full wave rectifier, the output of which is returned to the multiplier. Any second harmonic frequencies of the FM carrier signal generated by the full wave rectifier are recombined by the multiplier in an attempt to produce additional third harmonics of the carrier signal which tend to cancel the third harmonic frequencies produced from the carrier signal directly through the multiplier. However, besides requiring additional circuitry which increases circuit complexity as well as production costs, this prior art summing technique is not exact and may also produce higher order harmonics of the carrier signal that can distort the reproduced audio signal.
Thus, there is a need for a circuit and method that can be used to linearize the output signal generated from quadrature detectors of FM/IF receivers in order to reduce third harmonic distortion.